The present invention relates to a composite amplifier of the type that includes a main power amplifier and an auxiliary power amplifier, which are connected to a load over a Doherty output network.
In cellular base stations, satellite communications and other communications and broadcast systems, many radio frequency (RF) carriers, spread over a large bandwidth, are amplified simultaneously in the same high power amplifier (HPA alt. PA). For the power amplifier this has the effect that the instantaneous transmit power will vary very widely and very rapidly. This is because the sum of many independent RF carriers (a multi-carrier signal) tends to have a large peak-to-average power ratio. It also tends to have an amplitude distribution similar to single-sideband band-limited Gaussian noise, which has a Rayleigh distribution.
The main difficulties in a PA are efficiency and linearity. A conventional class-B power amplifier has an efficiency proportional to the output amplitude and exhibits maximum DC to RF power conversion efficiency when it delivers its peak power to the load. Since the quasi-Rayleigh distribution of amplitudes in the summed transmit signal has a large difference between the average power and the peak power, the overall efficiency when amplifying such a signal in a conventional class-B amplifier is very low. For a quasi-Rayleigh distributed signal with a 10-dB peak-to-average power ratio, the efficiency of an ideal class-B amplifier is only 28%, see [1].
The linearity of an RF power amplifier is usually characterized by its AM-AM (AM=amplitude modulation) and AM-PM (PM=phase modulation) distortion characteristics. Non-linearities manifest themselves as cross-mixing of different parts of the signal, leading to leakage of signal energy into undesired channels. By restricting the signal to be transmitted to a smaller part of the total voltage swing, the linearity can be increased. However, this reduces the efficiency of the amplifier even further. The linearity of a power amplifier is also reduced greatly if the amplifier saturates (the output voltage is clipped). This lessens the chance of increasing efficiency by driving the amplifier into saturation, since the distortion would then reach unacceptable levels.
A method for increasing the efficiency of an RF power amplifier is described in [1]. This amplifier, called a Doherty amplifier, uses in its basic form a main amplifier (also called carrier amplifier) and an auxiliary amplifier (also called peaking amplifier). The load is connected to the auxiliary amplifier, and the main amplifier is connected to the load through an impedance inverter, usually a quarter-wavelength transmission line or an equivalent lumped network.
At low output levels only the main amplifier is active. When the output level climbs over the so-called transition point (usually at half the maximum output voltage), the auxiliary amplifier becomes active driving current into the load, and through the impedance inverting action of the quarter-wavelength transmission line, decreases the effective impedance at the output of the main amplifier, to keep the main amplifier at a constant (peak) voltage. This is called negative load-pulling, and means that for the levels above the transition point, the main amplifier operates at maximum efficiency. At the same time, the auxiliary amplifier sees an increasing load, which is called positive load-pulling. The result is an approximately linear output power to input power relationship, but with a higher efficiency than a traditional amplifier.
Below the transition point, the auxiliary amplifier is shut off, and the main amplifier sees a higher (usually two times higher) load impedance than the impedance at peak power, which increases (doubles) its efficiency in this region also. The power lost in the auxiliary amplifier decreases the total efficiency slightly at levels above the transition point, but this action is small and negligible compared to the efficiency gained by using this technique.
The transition point can also be shifted, so that the auxiliary amplifier kicks in at a lower or higher power level. The power efficiency for an ideal Doherty amplifier, whose transition point is optimized for a quasi-Rayleigh distributed signal with a 10-dB peak-to-average power ratio, is increased to 60%, which is very high compared to the efficiency of an ideal class-B amplifier (28%), see [1].
Several patents have been granted for Doherty amplifiers, usually with small differences from what is described in [1], for example [2, 3, 4, 5]. The Doherty concept has also been extended to multi-stage variants [1, 4, 5]. This allows the efficiency to be kept higher over a broader range of output power levels.
A common feature of the prior art Doherty amplifiers described in [1, 2, 3, 4, 5] and also in [6, chapter 8, pp. 225-239] is that they all have the auxiliary amplifiers completely shut off below the transition point(s).
Due to mismatches between the phases and impedances of the amplifiers and due to nonlinear capacitances and resistances, distortion is generated at the transition point. The main amplifier is also expected to go into saturation [1], which changes a lot of characteristics of the amplifier. Saturation is generally known as a point where a lot of distortion is generated [6, chapter 7, pp. 179-218].
The main difference between the distortion effects in a Doherty amplifier and those encountered in a traditional power amplifier, such as a class-B, AB, or A, is that they occur right inside the range of power levels that are most often encountered. This is evident by looking at FIG. 9 of [2], in which the intermodulation distortion starts to rise very sharply at a point about 6 dB down from peak power. For traditional power amplifiers, most distortion is generated in the peak-power end, which is much less occupied for multi-carrier signals. The statement is even more true for the optimized Doherty amplifier, where the transition point is moved closer to the maximum of the Rayleigh distribution.
The distortion that is generated at the transition point is also of high order, that is, when described by a distortion polynomial, a polynomial with many coefficients (and hence powers of the voltage) is needed to describe it. This implies that if we want to correct for this distortion by applying an inverse function at the input of the amplifier, so called pre-distortion, a high order is needed also for the pre-distortion. This means that the bandwidth of the pre-distorter must be very large, which is a great problem if digital processing is used for the pre-distorter. It is also a problem to keep a constant gain and phase through the whole up-converter chain, over the entire used bandwidth, if the bandwidth is very large.
The wide bandwidth of the output from the auxiliary amplifier is also a problem in itself, since stringent phase and impedance matching conditions must be met, over the whole used bandwidth, between the main and auxiliary amplifier. The impedance inverter is by nature a narrowband device, but the bandwidth of the output signal of the auxiliary amplifier is very large. The auxiliary amplifier signal components outside of the impedance inverter optimum will not have the correct phase and amplitude at the output of the carrier amplifier, which causes distortion.
The conclusion is that the distortion in a Doherty amplifier is more severe, and that it is harder to correct for this distortion, than in a traditional RF power amplifier.
An object of the present invention is to provide a new composite amplifier which retains most of the efficiency and also most of the simplicity of the Doherty amplifier concept, while achieving greater linearity and easier cooperation with linearity-enhancement techniques.
This object is achieved in accordance with the attached claims.
Briefly, the present invention changes the abrupt characteristic of the Doherty auxiliary amplifier attenuation function into a smooth, low-order characteristic with an extended (possibly all the way from zero to peak power) soft transition region.
Amplifiers in accordance with the present invention have a lower distortion, better cooperation with linearization equipment and narrower bandwidth of the signals in the impedance inverter than the prior art Doherty amplifier. At the same time, most of the efficiency of the Doherty amplifier can be retained.